Experience at the East Wilmington oil field has shown that discrepancies may be expected with control points calculated from the directional surveys of highly deviated wells, compared to similar points in nearby straight holes. Drop-type multiple-shot surveys commonly differ from wireline single-shot surveys of the same well. The “average angle” method of survey computation is used by THUMS to reduce the margin of error. Even with this refinement, multiple shot surveys in high-drift wells are thought to have a possible error of ±20 ft (6.1 m) vertically and ±100 ft (30.5 m) horizontally for a well of 6,000 ft (1,828.8 m) measured depth. Structure and isopach maps are affected by this error, and the development geologist must use individual judgment in eliminating or “averaging” suspect control points.

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